During FY17 we accomplished the following: 1) Analyses of IgH chromatin status in CD4+ CD8+ (DP) thymocytes were written up for publication. The key findings being reported are: a) The IgH locus is partially active in DP cells as evidenced by reduced levels of activating histone modifications and non-coding transcription. b) These changes correlate with reduced activity of the intron enhancer, E. c) Transcription factor binding to E is distinct between pro-B and DP cells, characterized especially by lack of binding of E2A and ETS proteins in DP cells. d) CTCF-dependent IgH locus compaction is absent in DP cells despite normal binding of CTCF throughout the locus. Lack of compaction correlated with reduced Rad21 recruitment to IgH CTCF sites in DP cells. e) DH recombination pattern was markedly different in DP cells. This was reflected in reduced utilization of 5DFL16.1 gene segment as well as skewed utilization of 3 DSP2 gene segments. These changes correlated with reduced looping between E and IGCR1 in DP cells. f) Recombination center at IgH was substantially reduced in DP cells compared to pro-B cells. 2) Collaborative studies with Dr. Stephen Desiderio were submitted for publication. 3) Recombinase-sufficient mouse strains that carry point mutations in E were tested for IgH function following immunization with T-dependent and T-independent antigens. These studies were done in collaboration with Drs. Robert Maul and Patricia Gearhart. No changes in class switch recombination were detected. Analysis of somatic hypermutation is ongoing.